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atmospheric path is the presence of turbulence. Extreme examples of this are
familiar to all of us - the shimmering of objects near the ground on a hot day or
the twinkling of stars and distant lights. Unfortunately turbulence may be a
problem in viewing even when it is not so readily detectable. After all, the fact
that objects shimmer is due to local refraction by the atmosphere which varies
with time. Even when they do not shimmer it is highly likely that there are
temporal fluctuations in apparent object position which must interfere with
viewing. More importantly there are also likely to be changes of refraction across
the wavefronts being received by the two eye pupils, with resultant degradation
of retinal images in much the same way as by aberrations in the optics.
When using visual aids such as binoculars this local refraction becomes much
more serious. In such cases the entrance pupil is much enlarged compared with
that of the eye (M times the eye pupil where M is the magnification of the visual
aid) with the result that any tendency to shimmer is largely integrated. However,
this results in a degraded image quality replacing the image motion. In addition
any resultant degradation is magnified in its effect by the instrument. Thus
turbulence becomes potentially very important when viewing through visual
aids. It is particularly a problem in astronomy.
It is the purpose of this chapter to bring to the reader’s attention the forms of
turbulence, the available data on modelling of turbulence in terms of image
quality and the methods available to attempt to measure it.
16.1 The nature of turbulence
16.2 Forms of degradation due to turbulence
16.3 Theoretical studies
16.4 Parameter effects
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